The present invention is in the field of integrated circuits. More particularly, the present invention provides a method and apparatus for desoldering electronic components from a substrate. Another embodiment of the present invention provides a method and apparatus for depositing underfill material between an electronic component and the substrate on which the electronic component is mounted.
Electronic components, such as integrated circuit chips, are commonly attached to a substrate (e.g., a printed circuit board (PCB) or printed circuit card (PCC)) with solder connections using a ball grid array (BGA), chip scale package (CSP), or direct chip attach (DCA) technique. Occasionally, an electronic component may be found to be defective, and will therefore have to be removed and replaced with a functional electronic component using a rework process. In a conventional rework process, the defective electronic component is removed by first heating the solder material, used to connect the component""s solder connectors to corresponding contact pads on the substrate, to its melting, or xe2x80x9creflowxe2x80x9d temperature. Then, the defective electronic component is pulled off the substrate and replaced.
In a conventional rework process, a stream of hot gas is typically directed toward the top of the electronic component. This method works well if the solder connections are only located around or near the periphery of the electronic component, or when there is a relatively large gap between the bottom of the electronic component and the substrate. Unfortunately, using currently available reflow methods, heat from the stream of hot gas is not effectively or evenly transmitted to solder connections located away from the periphery of the electronic component (e.g., near the center of the electronic component). This is especially problematic if the space between the electronic component and the substrate is small, thereby restricting the flow of hot gas from the periphery to the center of the electronic component.
Electronic components mounted on a substrate commonly require underfill to increase reliability, mechanical integrity, and to ensure adequate operational life. For example, an underfill material such as epoxy is commonly inserted between an electronic component and a substrate to cover the solder connections, thereby protecting the solder connections from corrosion causing fluids or gases, and mechanically strengthening the connection between the electronic component and the substrate. Further, the use of underfill reduces failure of the solder connections due to cycling stresses caused by differences in the coefficients of thermal expansion of the electronic component and the substrate. Thus, underfill provides a robust mechanical connection preventing damaging relative motion between the electronic component and the substrate.
Commonly, the underfilling is accomplished by depositing a bead of underfill material along one or more sides of the electronic component and allowing capillary action to pull the underfill material under the electronic component. Unfortunately, not only is the process relatively slow and may leave voids in the underfill, but also requires the underfill material to be very fluid in nature. Thus, restrictions are placed on the composition of the underfill material.
The present invention avoids the disadvantages of the prior art by providing an improved method and apparatus for removing an electronic component from a substrate. Also, it the current invention provides an improved method and apparatus for applying underfill between the electronic component and the substrate.
In accordance with the present invention, a rework nozzle apparatus is used to remove an electronic component from a substrate. The rework nozzle apparatus includes an outer tube, an inner shaft, baffles, a vacuum source, a hot gas source, and a water vapor port. The outer tube has a cross-sectional shape slightly larger than that of the electronic component. A first end of the outer tube contacts the substrate surface, encloses the electronic component, and provides an essentially gas tight seal. The inner shaft has a cross-sectional shape similar to the top surface of the electronic component. A first end of the inner shaft contacts, and essentially provides a gas tight seal against, the top surface of the electronic component. The first end of the inner shaft may include projections for locating the electronic component in the horizontal direction. Baffles are attached between the inner shaft and the outer tube to direct a flow of hot gas beneath the electronic component, and to provide a seal against the substrate adjacent two sides of the electronic component. The outer tube, inner shaft, and the baffles form two ducts. The first duct is used to carry and direct a stream of hot gas to a region under a first side of the electronic component. The second duct is used to apply a vacuum to a region under a second side of the electronic component to increase the flow of hot gas under the electronic component. The vacuum is provided to the second duct by a vacuum source such as a vacuum pump. Solder connections under the electronic component are heated to a reflow temperature allowing the electronic component to be removed from the substrate. In order to increase the heat capacity of the hot gas, thereby enhancing thermal transfer to the solder connections, water vapor, or other suitable substance, is added to the hot gas through a water vapor port.
The rework nozzle apparatus may additionally include a vertical positioning apparatus, a heating element, and a reversing valve. The vertical positioning apparatus provides vertical positioning relative to the inner shaft, by means of a drive system such as a linear motor or stepper motor. The vertical positioning apparatus is slidably attached to the inner shaft. Heat is applied by the heating element to the inner shaft, preventing the inner shaft from drawing heat away from the electronic component during the rework process. The reversing valve periodically switches the vacuum from the second duct to the first duct, and simultaneously switches the hot gas from the first duct to the second duct, effectively reversing the direction of flow of the stream of hot gas. At the same time, the water vapor is switched from a water vapor port on the first duct to a water vapor port on the second duct. Advantageously, the use of the reversing valve provides a more uniform heating of the solder connections.
In accordance with the present invention, an underfill nozzle apparatus is used to insert underfill material under the electronic component. Preferably, underfill material is deposited along three sides of the electronic component, and a vacuum is applied under the fourth side of the electronic component to draw the underfill material under the electronic component.
The underfill nozzle apparatus includes a vacuum tube and a vacuum source. A first end of the vacuum tube contacts the substrate surface and provides an essentially gas tight seal. A side of the vacuum tube contacting the electronic component has an opening sized according to the cross-sectional open area under the electronic component. A vacuum is drawn through this opening promoting the flow of the underfill material under the electronic component.
Another embodiment of the underfill nozzle apparatus includes a vacuum tube, a vacuum source, an underfill tube, an underfill material source, baffles, a heat generating apparatus, and a control system. A first side of the vacuum tube contacts a first side of the electronic component. A first side of the underfill tube contacts a second, opposing side of the electronic component. The first side of the vacuum tube and the first side of the underfill tube each include an opening sized according to the cross-sectional open area under the electronic component.
The vacuum tube includes a first end that contacts the substrate surface, and a second end that is connected to a vacuum source. The underfill tube includes a first end that contacts the substrate surface, and a second end that is connected to a source of underfill material. A series of baffles are used to couple the vacuum tube to the underfill tube, and to seal the openings under the remaining open sides of the electronic component.
The heat generating apparatus provides means for heating the electronic component and the underfill material in the underfill tube to reduce the effective viscosity of the underfill material. The reduced viscosity of the underfill material results in a faster flow rate of underfill material beneath the electronic component.
Another embodiment of an underfill nozzle apparatus in accordance with the present invention includes a vacuum tube. A through hole is provided in the substrate at a location under the electronic component. Underfill material is deposited along the periphery of the electronic component and a first end of the vacuum tube is placed over the substrate through hole. The first end of the vacuum tube contacts the substrate surface on the side opposite from the electronic component, forming an essentially gas tight seal. A vacuum source is connected to a second end of the vacuum tube to generate a vacuum in the vacuum tube, the substrate hole, and the space underneath the electronic component. This vacuum rapidly pulls the underfill material under the electronic component.
In another embodiment of the underfill nozzle apparatus, a vacuum tube surrounds the electronic component on a first side of the substrate. A through hole is provided in the substrate at a location under the electronic component. A first end of an underfill tube contacts the substrate surface on the side opposite the electronic component, and encloses the through hole. A second end of the underfill tube is connected to an underfill supply source that provides underfill material to the area under the electronic component via the underfill tube and through hole. The vacuum surrounding the electronic component causes the underfill to rapidly fill the space under the electronic component.